U.S. patent application number 15/838499 was filed with the patent office on 2019-06-13 for partially disjoint equalization and carrier recovery.
This patent application is currently assigned to Huawei Technologies Co., Ltd.. The applicant listed for this patent is Nash'at Abughalieh, Syed Faisal Ali Shah, Chuandong Li. Invention is credited to Nash'at Abughalieh, Syed Faisal Ali Shah, Chuandong Li.
Application Number | 20190182082 15/838499 |
Document ID | / |
Family ID | 66697451 |
Filed Date | 2019-06-13 |
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United States Patent
Application |
20190182082 |
Kind Code |
A1 |
Abughalieh; Nash'at ; et
al. |
June 13, 2019 |
PARTIALLY DISJOINT EQUALIZATION AND CARRIER RECOVERY
Abstract
An apparatus in a signal receiver, such as an optical signal
receiver, is provided. An adaptive equalizer provides an equalized
output indicative of a received signal. A feedback component
receives the equalized output and provides feedback to the adaptive
equalizer. A carrier recovery component receives the equalized
output from the adaptive equalizer provides estimates of symbols.
The carrier recovery component is partially or fully disjoint from
the feedback component, thus removing the carrier recovery
component from equalizer the feedback loop. The feedback component
can include an initial carrier recovery component and a phase
rotation and detection component. The initial carrier recovery
component generates a carrier recovery output based on the
equalized output. The phase rotation and detection component
performs a phase rotation based on the carrier recovery output.
Inventors: |
Abughalieh; Nash'at;
(Kanata, CA) ; Ali Shah; Syed Faisal; (Cary,
NC) ; Li; Chuandong; (Ottawa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Abughalieh; Nash'at
Ali Shah; Syed Faisal
Li; Chuandong |
Kanata
Cary
Ottawa |
NC |
CA
US
CA |
|
|
Assignee: |
Huawei Technologies Co.,
Ltd.
Shenzhen
CN
|
Family ID: |
66697451 |
Appl. No.: |
15/838499 |
Filed: |
December 12, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 2201/02 20130101;
H04B 10/6164 20130101; H04L 25/03267 20130101; H04B 10/612
20130101; H04B 10/616 20130101; H04L 2025/03636 20130101; H04L
25/03273 20130101; H04L 25/03019 20130101; H04L 25/03885
20130101 |
International
Class: |
H04L 25/03 20060101
H04L025/03; H04B 10/61 20060101 H04B010/61 |
Claims
1. An apparatus in a signal receiver, the apparatus comprising: an
adaptive equalizer configured to receive an input indicative of a
received signal and to provide an equalized output indicative of a
corrected version of the received signal; a feedback component
configured to receive the equalized output from the adaptive
equalizer and provide feedback to the adaptive equalizer based on
the equalized output; and a fine carrier recovery component
configured to directly or indirectly receive the equalized output
from the adaptive equalizer and to provide estimates of symbols
carried in the received signal based on the equalized output,
wherein the fine carrier recovery component is partially or fully
disjoint from the feedback component.
2. The apparatus of claim 1, wherein the feedback component and the
fine carrier recovery component operate in parallel with each
other, and wherein at least a portion of data generated by the
feedback component is provided to the fine carrier recovery
component, at least a portion of data generated by the fine carrier
recovery component is provided to the feedback component, or
both.
3. The apparatus of claim 2, wherein the portion of data generated
by the feedback component and provided to the fine carrier recovery
component includes an estimate of phase offsets in the received
signal.
4. The apparatus of claim 2, wherein the portion of data generated
by the fine carrier recovery component and provided to the feedback
component includes an estimate of frequency of components of the
received signal.
5. The apparatus of claim 1, wherein the feedback component
performs an initial phase rotation on the equalized output and
provides the equalized output following the initial phase rotation
to the fine carrier recovery component.
6. The apparatus of claim 1, wherein the adaptive equalizer
operates on plural data inputs in parallel, and wherein the
feedback component operates on another plural data inputs in
parallel.
7. The apparatus of claim 6, wherein the fine carrier recovery
component operates on a third set of plural data inputs in
parallel.
8. The apparatus of claim 1, wherein the equalized output comprises
a first number N of symbols in parallel, the feedback component is
configured to operate on all of the N symbols in parallel, and the
fine carrier recovery component is configured to operate on a
second number M of the N symbols in parallel, wherein M is less
than N.
9. The apparatus of claim 1, wherein the feedback component
comprises: an initial carrier recovery component configured to
generate a carrier recovery output based on the equalized output
from the adaptive equalizer and an estimate of frequency components
of the received signal, the estimate received from the fine carrier
recovery component; a phase rotation and detection component
configured to receive the equalized output from the adaptive
equalizer and to detect symbols carried in the received signal
based on the equalized output following a phase rotation performed
on the equalized output, the phase rotation performed by the phase
rotation and detection component based on the carrier recovery
output generated by the initial carrier recovery component, the
detected symbols fed back directly or indirectly to the adaptive
equalizer.
10. The apparatus of claim 1, wherein the feedback component
comprises: an initial carrier recovery component configured to:
receive the equalized output from the adaptive equalizer and
generate an initial estimate of phase offset in the received signal
based on the equalized output; generate an integral of frequency
offset in the received signal based on an estimate of frequency of
components of the received signal, the estimate received from the
fine carrier recovery component; and generate a carrier recovery
output based on the initial estimate of phase offset and the
integral of frequency offset; a phase rotation and detection
component configured to receive the equalized output from the
adaptive equalizer and to: perform a phase rotation on the
equalized output based on the carrier recovery output of the
initial carrier recovery component; detect symbols carried in the
received signal based on the equalized output following the phase
rotation and provide a detector output indicative of the detected
symbols; and perform a phase de-rotation on the detector output,
the phase de-rotation opposing the phase rotation and performed
based on the carrier recovery output of the initial carrier
recovery component, the phase de-rotated detector output fed back
to the adaptive equalizer.
11. An apparatus in a signal receiver for providing feedback to an
adaptive equalizer of the signal receiver, the apparatus
comprising: an initial carrier recovery component configured to
generate a carrier recovery output based on equalized output
received from the adaptive equalizer and an estimate of frequency
components of the received signal; and a phase rotation and
detection component configured to receive the equalized output from
the adaptive equalizer and to detect symbols carried in the
received signal based on the equalized output following a phase
rotation performed on the equalized output, the phase rotation
performed based on the carrier recovery output generated by the
initial carrier recovery component, the detected symbols fed back
directly or indirectly to the adaptive equalizer.
12. The apparatus of claim 11, wherein the initial carrier recovery
component is further configured to: generate an initial estimate of
phase offset in the received signal based on the equalized output;
generate an integral of frequency offset in the received signal
based on the estimate of frequency of components of the received
signal; and generate the carrier recovery output based on the
initial estimate of phase offset and the integral of frequency
offset.
13. The apparatus of claim 12, wherein generating the integral of
frequency offset comprises integrating the estimate of frequency
components using a ramp function.
14. The apparatus of claim 12, wherein generating the carrier
recovery output comprises operating a numerically controlled
oscillator based on the initial estimate of phase offset and the
integral of frequency offset.
15. The apparatus of claim 11, wherein the phase rotation and
detection component is further configured to: perform the phase
rotation on the equalized output; provide a detector output
indicative of the detected symbols; and perform a phase de-rotation
on the detector output, the phase de-rotation opposing the phase
rotation and performed based on the carrier recovery output of the
initial carrier recovery component, the phase de-rotated detector
output fed back to the adaptive equalizer as the detected
symbols.
16. The apparatus of claim 11, wherein the estimate of frequency
components is received from another component of the signal
receiver.
17. The apparatus of claim 16, wherein the other component of the
signal receiver is a fine carrier recovery component configured to
receive the equalized output from the adaptive equalizer and to
provide estimates of symbols carried in the received signal based
on the equalized output, wherein the fine carrier recovery
component is partially or fully disjoint from the apparatus.
18. The apparatus of claim 17, further configured to provide an
estimate of phase offsets in the received signal to the fine
carrier recovery component.
19. The apparatus of claim 11, further comprising a fine carrier
recovery component separate from the initial carrier recovery
component and the phase rotation and detection component, wherein
the phase rotation and detection component is configured to provide
the equalized output following the phase rotation as symbol input
to the fine carrier recovery component.
20. The apparatus of claim 11, wherein the apparatus operates on
plural data inputs in parallel.
21. The apparatus of claim 11, wherein the equalized output
comprises a first number N of symbols in parallel, and the
apparatus is configured to operate on all of the N symbols in
parallel, and the fine carrier recovery component is configured to
operate on a second number M of the N symbols in parallel, wherein
M is less than N.
22. A method for operating a signal receiver, comprising:
providing, by an adaptive equalizer, an equalized output indicative
of a corrected version of a received signal; receiving, by a
feedback component, the equalized output from the adaptive
equalizer; providing, by the feedback component, feedback to the
adaptive equalizer based on the equalized output; directly or
indirectly receiving, by a fine carrier recovery component separate
from the feedback component, the equalized output from the adaptive
equalizer; and providing, by the fine carrier recovery component,
estimates of symbols carried in the received signal based on the
equalized output.
23. The method of claim 22, further comprising one or both of:
providing at least a portion of data generated by the feedback
component to the fine carrier recovery component; and providing at
least a portion of data generated by the fine carrier recovery
component to the feedback component.
24. The method of claim 22, further comprising, by the feedback
component, performing an initial phase rotation on the equalized
output and providing the equalized output following the initial
phase rotation to the fine carrier recovery component.
25. The method of claim 22, further comprising: generating, by an
initial carrier recovery sub-component of the feedback component, a
carrier recovery output based on the equalized output from the
adaptive equalizer and an estimate of frequency components of the
received signal, the estimate received from the fine carrier
recovery component; by a phase rotation and detection sub-component
of the feedback component: receiving the equalized output from the
adaptive equalizer; performing a phase rotation on the equalized
output; and detecting symbols carried in the received signal based
on the equalized output following the phase rotation, wherein the
phase rotation is performed based on the carrier recovery output
generated by the initial carrier recovery component, and wherein
the detected symbols are fed back directly or indirectly to the
adaptive equalizer.
26. The method of claim 22, further comprising: by an initial
carrier recovery sub-component of the feedback component: receiving
the equalized output from the adaptive equalizer; generating an
initial estimate of phase offset in the received signal based on
the equalized output; generating an integral of frequency offset in
the received signal based on an estimate of frequency of components
of the received signal, the estimate received from the fine carrier
recovery component; and generating a carrier recovery output based
on the initial estimate of phase offset and the integral of
frequency offset; and by a phase rotation and detection
sub-component of the feedback component: receiving the equalized
output from the adaptive equalizer; performing a phase rotation on
the equalized output based on the carrier recovery output of the
initial carrier recovery component; detecting symbols carried in
the received signal based on the equalized output following the
phase rotation and provide a detector output indicative of the
detected symbols; performing a phase de-rotation on the detector
output, the phase de-rotation opposing the phase rotation and
performed based on the carrier recovery output of the initial
carrier recovery component; and feeding back the phase de-rotated
detector output to the adaptive equalizer.
27. A method for operating a signal receiver, comprising: by an
initial carrier recovery component, generating a carrier recovery
output based on: equalized output received from an adaptive
equalizer of the signal receiver, and an estimate of frequency
components of the received signal; by a phase rotation and
detection component: receiving the equalized output from the
adaptive equalizer; performing a phase rotation on the equalized
output, based on the carrier recovery output generated by the
initial carrier recovery component; detecting symbols carried in
the received signal based on the equalized output following the
phase rotation; and feeding back the detected symbols directly or
indirectly to the adaptive equalizer.
28. The method of claim 27, further comprising, by the initial
carrier recovery component: generating an initial estimate of phase
offset in the received signal based on the equalized output;
generating an integral of frequency offset in the received signal
based on the estimate of frequency of components of the received
signal; and generating the carrier recovery output based on the
initial estimate of phase offset and the integral of frequency
offset.
29. The method of claim 27, further comprising, by the phase
rotation and detection component: performing the phase rotation on
the equalized output; providing a detector output indicative of the
detected symbols; performing a phase de-rotation on the detector
output, the phase de-rotation opposing the phase rotation and
performed based on the carrier recovery output of the initial
carrier recovery component; and feeding back the phase de-rotated
detector output to the adaptive equalizer as the detected
symbols.
30. The method of claim 27, wherein the estimate of frequency
components is received from a fine carrier recovery component
configured to receive the equalized output from the adaptive
equalizer and to provide estimates of symbols carried in the
received signal based on the equalized output, wherein the fine
carrier recovery component is partially or fully disjoint from the
apparatus.
31. The method of claim 30, further comprising providing an
estimate of phase offsets in the received signal to the fine
carrier recovery component.
32. The method of claim 27, further comprising providing, by the
phase rotation and detection component, the equalized output
following the phase rotation as symbol input to a fine carrier
recovery component, wherein the fine carrier recovery is component
separate from the initial carrier recovery component and the phase
rotation and detection component.
33. The method of claim 27, further comprising operating on plural
data inputs in parallel by one or both of the initial carrier
recovery component and the phase rotation and detection
component.
34. The method of claim 27, wherein the equalized output comprises
a first number N of symbols in parallel, the method further
comprising operating on all of the N symbols in parallel, and
wherein a fine carrier recovery component is configured to operate
on a second number M of the N symbols in parallel, wherein M is
less than N.
Description
FIELD OF THE INVENTION
[0001] The present application pertains to the field of
communication signal reception and processing. More particularly,
the present application relates to a method and apparatus for
performing channel equalization and carrier recovery in a signal
receiver such as a coherent optical receiver.
BACKGROUND
[0002] The increase in bandwidth demand for optical links, such as
the links found in long haul optical networks, can be satisfied by
increasing the capacity of optical link. Increasing the baud rate
is among the methods that can be used to increase the capacity of
the optical links. Increasing the baud rate requires an increase in
the clocking speed of the Digital Signal Processing (DSP) chip,
except if the data samples are processed in bigger block sizes
(parallelizing the data path). There is also demand for energy
consumption reduction in the DSP chip, which may be obtained by
reducing clock speed and increasing DSP processer block size. The
drawback of increasing the DSP processor block size is the
discontinuity in the adaptation of the DSP loops. It can be
challenging to retain the performance of the DSP algorithms while
reducing the loops update rate.
[0003] Coherent optical receivers typically use adaptive equalizers
to compensate for time-varying polarization, mode dispersion
effects, and inter-symbol interference (ISI). Performance of
coherent optical receivers often relies heavily on carrier recovery
(CR) methods. The interplay between adaptive equalization and CR
has led to two types of coherent receiver architectures: blind
equalization with independent CR; and joint equalization and
CR.
[0004] For blind equalization with independent CR, the adaptive
equalizer can use a constant modulus algorithm (CMA) for equalizer
convergence. In this case the equalizer is decoupled from the
carrier recovery and potentially has low complexity. However, the
CMA typically exhibits a slow convergence rate, and this scheme can
be sensitive to higher order modulations, such as 16QAM or
higher.
[0005] For joint equalization and CR, the equalizer can be adapted
in a decision directed (DD) manner, for example using a Least Mean
Squares (LMS) algorithm. This approach exhibits faster convergence
and better performance than blind equalizers. However, for
implementation, the equalizer typically requires full knowledge of
signal phase offsets to update its filter coefficients.
Furthermore, due to its limited speed, the presence of the CR
operation the adaptive equalizer feedback loop limits the speed of
equalizer adaptation.
[0006] Therefore there is a need for an adaptive equalizer in a
signal receiver, for example in an optical modem, that is not
subject to one or more limitations of the prior art, such as
decreasing performance with increasing data block size.
[0007] This background information is provided to reveal
information believed by the applicant to be of possible relevance
to the present invention. No admission is necessarily intended, nor
should be construed, that any of the preceding information
constitutes prior art against the present invention.
SUMMARY
[0008] An object of the present invention is to provide a method
and apparatus for performing channel equalization and carrier
recovery in a signal receiver such as a coherent optical receiver.
The signal receiver can be one which employs parallelized
processing of received data by processing a sequence of blocks,
each block containing multiple received symbols which are processed
concurrently. In accordance with an aspect of the present
invention, two operations, namely adaptive channel equalization and
carrier recovery, are performed in a partially disjoint manner. The
adaptive channel equalization employs a feedback loop which
involves a phase rotation and detection operation. The phase
rotation and detection operation relies on output of an initial
phase offset estimation operation which run on the filter output
and gives rough phase offset estimate that is used, together with
the frequency offset that can be estimated from the adaptive filter
output or calculated in the carrier recovery from previous filter
outputs, as input information to perform phase rotation on the
adaptive filter output to correct partially phase and frequency
offset, prior to symbol detection in the detection operation. The
initial phase offset estimation operation can be part of what is
referred to herein as an initial carrier recovery operation or
component. The initial carrier recovery operation (and associated
initial phase and/or frequency offset correction) is performed
separately from a final (or fine), more fully functional carrier
recovery operation, which results in the disjoint nature of the two
operations. However, in some embodiments, data generated by the
initial carrier recovery operation may be passed to and used by the
fine carrier recovery operation (also sometimes referred to as
final carrier recovery), data generated by the fine carrier
recovery operation is passed to and used by the initial carrier
recovery operation, or both. The data may include carrier phase
estimates, carrier frequency estimates, or both. Alternatively the
data may include adaptive filter output after being subjected to an
initial phase rotation, this data being passed to the fine carrier
recovery operation. This sharing of data results in the partial
disjointness (rather than full disjointness) of the adaptive
equalization and carrier recovery operations.
[0009] In a related aspect of the present invention, and adaptive
channel equalization method and apparatus is provided which employs
a feedback loop, the feedback loop involving a phase rotation and
detection operation. The phase rotation and detection operation
relies on output of an initial carrier recovery operation. The
initial carrier recovery operation is operatively coupled with a
separate, fine carrier recovery operation. In some embodiments, the
initial carrier recovery operation provides estimates (e.g. phase
estimates) to the fine carrier recovery operation. In some
embodiments, the fine carrier recovery operation provides estimates
(e.g. carrier frequency estimates) to the initial carrier recovery
operation. In some embodiments, the initial carrier recovery
operation provides adaptive filter output, after being subjected to
an initial phase rotation, to the fine carrier recovery
operation.
[0010] In accordance with an embodiment of the present invention,
there is provided an apparatus in a signal receiver. The apparatus
includes an adaptive equalizer configured to receive input
indicative of a received signal and to provide an equalized output
indicative of a corrected version of the received signal. The
apparatus further includes a feedback component configured to
receive the equalized output from the adaptive equalizer and
provide feedback to the adaptive equalizer based on the equalized
output. The apparatus further includes a fine carrier recovery
component configured to directly or indirectly receive the
equalized output from the adaptive equalizer and to provide
estimates of symbols carried in the received signal based on the
equalized output, wherein the fine carrier recovery component is
partially or fully disjoint from the feedback component.
[0011] In accordance with another embodiment of the present
invention, there is provided an apparatus in a signal receiver for
providing feedback to an adaptive equalizer of the signal receiver.
The apparatus includes an initial carrier recovery component
configured to generate a carrier recovery output based on equalized
output received from the adaptive equalizer and an estimate of
frequency components of the received signal. The apparatus further
includes a phase rotation and detection component configured to
receive the equalized output from the adaptive equalizer and to
detect symbols carried in the received signal based on the
equalized output following a phase rotation performed on the
equalized output, the phase rotation performed based on the carrier
recovery output generated by the initial carrier recovery
component, the detected symbols fed back directly or indirectly to
the adaptive equalizer.
[0012] In accordance with another embodiment of the present
invention, there is provided a method for operating a signal
receiver. The method includes providing, by an adaptive equalizer,
an equalized output indicative of a corrected version of a received
signal. The method further includes receiving, by a feedback
component, the equalized output from the adaptive equalizer. The
method further includes providing, by the feedback component,
feedback to the adaptive equalizer based on the equalized output.
The method further includes directly or indirectly receiving, by a
fine carrier recovery component separate from the feedback
component, the equalized output from the adaptive equalizer. The
method further includes providing, by the fine carrier recovery
component, estimates of symbols carried in the received signal
based on the equalized output.
[0013] In accordance with another embodiment of the present
invention, there is provided a method for operating a signal
receiver. The method includes, by an initial carrier recovery
component, generating a carrier recovery output based on: equalized
output received from an adaptive equalizer of the signal receiver,
and an estimate of frequency components of the received signal. The
method further includes, by a phase rotation and detection
component: receiving the equalized output from the adaptive
equalizer; performing a phase rotation on the equalized output,
based on the carrier recovery output generated by the initial
carrier recovery component; detecting symbols carried in the
received signal based on the equalized output following the phase
rotation; and feeding back the detected symbols directly or
indirectly to the adaptive equalizer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Further features and advantages of the present invention
will become apparent from the following detailed description, taken
in combination with the appended drawing, in which:
[0015] FIG. 1 is a simplified schematic diagram of an embodiment of
a communication system where embodiments of the present disclosure
may operate.
[0016] FIG. 2 is a simplified schematic diagram of an adaptive
filter.
[0017] FIG. 3 is a simplified schematic diagram of a decision
directed (DD) adaptive filter.
[0018] FIG. 4 is a simplified schematic diagram of an embodiment of
a decision directed (DD) adaptive filter/equalizer of the prior
art, including an adaptive filter and a carrier recovery component
of the prior art.
[0019] FIG. 5 is a schematic diagram of an embodiment of FIG. 4,
showing parallel data processing, according to the prior art.
[0020] FIG. 6 illustrates an apparatus having partially disjoint
feedback and carrier recovery components, in accordance with
embodiments of the present invention.
[0021] FIG. 7 illustrates a feedback component of an apparatus
provided in accordance with an embodiment of the present invention,
the feedback component having interacting initial carrier recovery
and phase rotation and detection components.
[0022] FIG. 8 illustrates details of the feedback component of FIG.
7, in accordance with a further embodiment of the present
invention.
[0023] FIG. 9 illustrates an apparatus provided in accordance with
another embodiment of the present invention, the apparatus
incorporating the components of FIG. 8.
[0024] FIG. 10 illustrates an apparatus provided in accordance with
another embodiment of the present invention, corresponding to an
alternative configuration of the feedback component and fine
carrier recovery component.
[0025] FIG. 11 illustrates a method for operating an adaptive
equalizer, in accordance with embodiments of the present
invention.
[0026] FIGS. 12A to 12C illustrate a method for operating a signal
receiver comprising an adaptive equalizer, in accordance with
another embodiment of the present invention.
[0027] FIG. 13 illustrates an apparatus provided in accordance with
another embodiment of the present invention, corresponding to an
alternative configuration of the initial carrier recovery
component.
DETAILED DESCRIPTION
[0028] Disclosed herein are at least one method, apparatus, and/or
system implementing partially disjoint decision directed adaptive
channel equalization and carrier recovery. A receiver in a
communication system comprises an adaptive equalizer and a feedback
component. In various embodiments, the adaptive equalizer includes
an adaptive filter (such as a Finite Impulse Response (FIR)
filter), signal error calculator, and a coefficient update
calculator. In various embodiments, the feedback component includes
an initial carrier recovery component and a phase rotation and
detection component. The initial carrier recovery component may
include an initial phase estimator, a ramp generator (also referred
to as a phase ramp calculator), and/or a numerically controlled
oscillator. The phase rotation and detection component may include
one or more phase rotators and one or more detectors. The
above-mentioned components each process multiple data inputs in
parallel or serially. In practice, various components and
sub-components are provided as electronic circuits, the definitions
and boundaries of which may not necessarily be clear and distinct.
Thus, the arrangement, naming and categorization of components as
described herein is used for purposes of clarity and is not
necessarily intended to be limiting.
[0029] The receiver may include an analog-to-digital converter
(ADC) providing samples in parallel, or the ADC may provide serial
samples which are converted to blocks of parallel samples.
Receiving and processing samples in parallel may enable the
receiver to operate at higher data rates. The parallel samples may
be processed using the adaptive filter (e.g. the adaptive channel
equalization filter of the adaptive equalizer) first, then
converted to symbols, or the samples may be converted to symbols
first then equalized. In either case, the output of the adaptive
filter is assumed to be blocks of symbols. These blocks of symbols
may at least intermittently contain a transmitted training
sequence. This training sequence may be used by the initial carrier
recovery component to estimate initial phase offsets. The initial
phase offsets, estimated for example using the training sequence,
can in turn be used to partially correct phase offsets of adaptive
filter outputs until the next training sequence is received. A
detector (e.g. within the phase rotation and detection component)
is used to detect originally transmitted symbols by analysis of the
partially phase and frequency corrected adaptive filter output.
Output of the detector can be de-rotated in phase and the result
may be used with the filter output to estimate the error in the
filter output. The coefficient update calculator may use the
estimated error to update the adaptive filter's coefficients, so
that the adaptive filter processes subsequently received data using
filter coefficients provided by the coefficient update calculator.
Carrier recovery components, such as a fine carrier recovery
component, may use the initial phase and frequency estimates, along
with the adaptive filter output, to fine track the phase offset in
the symbols and correct it, separately from the adaptive
equalizer's feedback loop.
[0030] Optical receivers rely on parallel processing to achieve
high throughput requirements, for example on the order of 100 Gbps.
The demand for higher throughput is pushing technology towards the
use of higher order QAM modulation and high performance receivers
that require sophisticated equalization and CR algorithms. In
general, coherent optical receivers are more sensitive to phase
noise changes with time than to ISI changes with time. It is
observed herein that the equalizer can process larger blocks of
data before it needs to update the coefficients, while the CR
operation has to process data in smaller blocks to keep track of
phase changes. While block equalizers can reduce system complexity
through frequency domain implementations, the presence of the CR
operation in the feedback loop limits the equalizer's adaptation
speed and continuity.
[0031] According to embodiments of the present invention,
therefore, carrier recovery operations are separated into two
stages. In the first stage, referred to as initial carrier
recovery, initial phase estimation and phase ramp calculations are
performed in order to provide a relatively coarse initial phase
estimate which is used for adaptive equalization. The initial phase
estimate can be obtained through a variety of methods, for example
using training sequences. The phase ramp calculation can be
performed based on a prior frequency estimate. The prior frequency
estimate can be generated based on a previously received and
processed data block. In the second stage, referred to as fine
carrier recovery, finer-grained carrier recovery is performed using
a feedback or feedforward architecture. The results of the initial
carrier recovery stage are used to provide primary feedback to the
adaptive equalizer, while the results of the fine carrier recovery
stage are passed to a decoder but not necessarily used to provide
feedback to the adaptive equalizer. That is, the fine carrier
recovery stage can be decoupled from the adaptive equalizer
feedback loop, while maintaining the use of decision directed
equalization. However, in some embodiments output from the fine
carrier recovery stage can still be used in the adaptive feedback
equalizer loop, for example in a slower feedback loop operating in
tandem with the primary feedback loop.
[0032] Because the initial carrier recovery operation exhibits
lower delay than the fine carrier recovery operation, decision
directed adaptive equalization can be implemented with lower
feedback loop delay than would be possible if only the fine carrier
recovery were included in the back loop. It has been observed
through simulation that the benefits gained from this reduced
feedback loop delay can outweigh the drawbacks due to reduced
quality of output from the initial carrier recovery stage (relative
to the fine carrier recovery stage). As such, in embodiments of the
present invention, performing partially disjoint equalization and
carrier recovery potentially results in improved performance over
prior art fully joint equalization and carrier recovery
implementations. It is currently believed that this is due at least
in part to reduced delays in the adaptive equalization feedback
loop, especially for block-based transform domain adaptive
equalization implementations.
[0033] In comparison to fully blind equalization, the initial
carrier recovery operation obtains at least a coarse estimate of
phase errors. This allows for a form of decision directed adaptive
equalization that typically exhibits improved performance over
fully blind equalization.
[0034] It is noted that the adaptive equalizer itself can
potentially operate in substantially the same manner as prior art
adaptive equalizers, except that output samples are phase rotated
using phase estimates from the initial carrier recovery stage
only.
[0035] FIG. 1 is a schematic diagram of a communication system 100
with respect to which embodiments of the present disclosure may
operate. The communication system 100 may be any communication
system that includes, but is not limited to, optical networks,
wired networks and/or wireless networks. The communication system
100 may comprise a transmitting node 102, a communication channel
104, and a receiver 106.
[0036] FIG. 2 is a schematic view of a system 200 comprising a
communication channel 202 with the transfer function H(n) and an
adaptive filter 204. The channel outputs y(n), which is generated
by applying the transfer function H(n) to the input (transmitted
signal) x(n). The adaptive filter 204 provides an estimate of the
transmitted signal x(n) by inverting the linear operation H(n)
applied by the channel 202. The estimate of x(n) is also used to
tune in the adaptive filter 204.
[0037] The adaptive filter can be used to perform different
functions, including but not limited to: samples interpolation,
interference cancelation and/or equalization.
[0038] The adaptive filter 204 can be realized using an FIR filter
which convolves the input sequence y(n) with the adaptive filter
coefficients w(n). The output of the adaptive filter can be the
estimate {circumflex over (x)}(n)=.SIGMA..sub.m=0.sup.Ly(m)w(n-m)
where L is the length of the adaptive filter. The filter operation
can be performed in the frequency domain by transforming the input
signal to the frequency domain and multiplying the input signal by
the frequency domain version of the filter coefficients, then
transforming the result back to time domain.
[0039] The filter coefficients can be adapted based on the adaptive
filter output alone. This is referred to as blind adaptation.
Alternatively the filter coefficients can be adapted using the
error in the output of the adaptive filter compared with estimates
of the transmitted symbols. This is referred to as decision
directed adaptation. FIG. 3 illustrates a high-level representation
of a decision directed adaptive filter 300.
[0040] In the decision directed adaptive filter 300 of FIG. 3, the
adaptive filter coefficients are updated using a function of the
error between the output of a primary adaptive filter 302 and the
estimate of the transmitted data. The output of the primary
adaptive filter 302 may pass through linear processing blocks 304
which perform signal operations represented by G(n). The signal
operations may include phase and/or frequency offset corrections.
After the linear processing, the symbols may be provided to a
decision device 306 that estimates transmitted symbols. The
decision device may be implemented as a symbol detector. Another
processing block 308, which performs operations G.sup.-1(n) which
are the inverse of those applied by the linear processing block
304, may receive and process the output of the decision device 306.
The output of this processing block 308 is then provided to an
error signal calculator block 310, which determines and provides an
error signal for use in adjusting the primary adaptive filter
302.
[0041] FIG. 4 illustrates a decision directed adaptive equalizer
400, according to the prior art. The output of an adaptive filter
402 of an adaptive equalizer component 418 may be passed through a
fully functioning carrier recovery component 420. An error signal
calculator 414 (also referred to as an error signal generator) then
generates an error signal based on output of the carrier recovery
component 420 and/or output of the adaptive filter 402. The filter
coefficients are updated, by the coefficient update calculator 416,
based on a function of the generated error signal and/or the filter
input. Although subsequent figures do not explicitly show all
inputs to the error signal calculator and coefficient update
calculator, it should be understood that the error signal
calculator and coefficient update calculator provided according to
embodiments of the present invention operate similarly, and are
based on similar information. The carrier recovery component 420
also provides output to a decoder 408 which decodes the corrected
symbols from the carrier recovery component and provides output
data.
[0042] The carrier recovery component 420 operates as follows. The
phase rotator 404 may correct phase offset of the adaptive filter
output symbols with the phase offset estimated from the previous
symbols. The detector 406 may detect the transmitted symbols. A
phase and frequency offset detector 412 may use the error between
inputs and outputs of the detector to estimate phase and frequency
offset. A phase de-rotator 410 may reverse the phase rotator
operation to facilitate calculation, by the error signal calculator
414, of the error in adaptive filter output. The carrier recovery
component 420 may run different algorithms to correct phase and
frequency offsets. The carrier recovery component 420 as
illustrated shows one of the different techniques for correcting
phase and frequency offset.
[0043] FIG. 5 illustrates an implementation for the apparatus in
FIG. 4, according to the prior art. Notably, the carrier recovery
component 420 operates on fewer bits at a time (M bits) than the
adaptive equalizer or decoder (N>M bits). Converters 510, such
as partial serializers/deserializers or block size converters are
provided at the inputs and outputs of the carrier recovery
component 420 to facilitate this. However, as discussed above, the
presence of the carrier recovery in the feedback loop can be
limiting to the adaptive equalizer operation.
[0044] Having reference now to FIG. 6, embodiments of the present
invention provide for an apparatus 600 in a signal receiver,
comprising at least an adaptive equalizer 610 and a feedback
component 630. The adaptive equalizer 610 is configured to receive
input 612 indicative of a received signal and to provide an
equalized output 614 indicative of a corrected (e.g. adjusted to
compensate for channel distortion, fading, or other effects)
version of the received signal. The input 612 may be received from
an analog to digital converter 605, for example. The equalized
output 614 may be derived from applying filtering, such as FIR
filtering, to the input 612, where the filter coefficients are
updated using feedback. The feedback component 630 is configured to
receive the equalized output 614 from the adaptive equalizer and to
provide feedback 632 (for updating the filter coefficients) back to
the adaptive equalizer 610 based on the equalized output 614.
[0045] In various embodiments, the apparatus 600 further includes a
fine carrier recovery component 670 configured to directly or
indirectly receive the equalized output 614 from the adaptive
equalizer 610, and to provide estimates 672 of symbols carried in
the received signal based on the equalized output. The carrier
recovery component 670 is partially disjoint, that is, operates
independently, from the feedback component 630, as will be
explained below. The symbol estimates 672 may be provided to a
decoder 690. The fine carrier recovery component 670 is configured
to estimate and correct for phase and frequency offsets in the
equalized output 614. This may include phase and frequency offsets
not corrected for by the feedback component 630, or phase and
frequency offsets corrected for by the feedback component 630 but
not passed to the fine carrier recovery component. The fine carrier
recovery component 670 is located outside of the adaptive filter
loop. Decoupling the fine carrier recovery component 670 from the
adaptive filter loop can reduce delay in adaptive filter loop while
potentially improving the performance of the adaptive filter loop
and improving adaptation speeds of the equalizer.
[0046] In some embodiments, the fine carrier recovery component 670
directly receives the equalized output 614 from the adaptive
equalizer 610. The fine carrier recovery component 670 may
partially serialize the equalized output prior to processing, for
example by converting blocks of N symbols into smaller blocks of M
symbols. As such, the feedback component 630 and the fine carrier
recovery component 670 can operate in parallel with each other.
[0047] Furthermore, in some embodiments, at least a portion of data
616 generated by the feedback component, such as an estimate of
phase offsets in the received signal, is provided to the fine
carrier recovery component. Additionally or alternatively, in some
embodiments, at least a portion of data 617 generated by the fine
carrier recovery component, such as an estimate of frequency of
components of the received signal, is provided to the feedback
component. The feedback component 630 and the fine carrier recovery
component 670 may include communication links, such as data lines,
to facilitate this data exchange.
[0048] In other embodiments, the fine carrier recovery component
670 indirectly receives the equalized output 614 from the adaptive
equalizer 610 in the following manner. The feedback component 630
performs an initial phase rotation on the equalized output 614 and
provides the equalized output following the initial phase rotation
615 to the carrier recovery component 630. In some such
embodiments, no other data is exchanged between the feedback
component 630 and the fine carrier recovery component 670. In other
such embodiments, other data may also be exchanged between these
two components.
[0049] As such, in various embodiments, the fine carrier recovery
component may operate on the equalized output 614 or on the
equalized output following the initial phase rotation 615. In some
embodiments, the fine carrier recovery component may use initial
phase estimate data 616 provided by an initial phase estimator of
the feedback component 630.
[0050] In various embodiments, one, some, or all of the adaptive
equalizer 610, the feedback component 630, and the fine carrier
recovery component 670 may operate on plural data inputs in
parallel. For example, the adaptive equalizer 630 may operate
concurrently on each input, of a block of N inputs, received in
parallel from the ADC 605. The feedback component 630 may operate
concurrently on each input, of a block of N inputs, received in
parallel from the adaptive equalizer 610. As described below, the
feedback component 630 may comprise both an initial carrier
recovery component and a phase rotation and detection component. In
some embodiments, at least the phase rotation and detection
component operates on plural data inputs in parallel. In some
embodiments both the initial carrier recovery component and the
phase rotation and detection component operate on plural data
inputs in parallel.
[0051] The data path from the adaptive equalizer through the phase
rotation and detection component and back to the adaptive equalizer
is referred to as the adaptive filter loop. For example, the
adaptive filter loop involves a looped data path from the adaptive
filter through the phase rotator, detector, phase de-rotator, error
signal calculator, and coefficient update calculator. In various
embodiments, the parallel vector width can be the same through all
the portions of the adaptive filter loop. That is, each component
in the adaptive filter loop can receive, operate on, and output
blocks of N inputs in parallel.
[0052] The fine carrier recovery component 670 may operate
concurrently on each input, of a block of M<N inputs, received
from the adaptive equalizer 610. Alternatively, the fine carrier
recovery component 670 may operate on a single input at a time. A
(partial or full) serializer, also referred to as a block size
converter, may receive and buffer a series of data blocks, each of
size N, provided by the adaptive equalizer. The serializer
re-allocates the buffered data into smaller data blocks of size
M<N, and the smaller data blocks are provided as input to the
fine carrier recovery component 670. Blocks of inputs are provided
in series, such that a first block of inputs is provided a first
time, a second block of inputs is provided at a second, later time,
etc. Provided that the serializer outputs the smaller data blocks
at a time-averaged rate of at least Nr/M, where r is the rate at
which the serializer receives data blocks, then a buffer overflow
condition can be avoided. If a large buffer is provided, the rate
at which the serializer outputs data blocks can be temporarily
lower than Nr/M.
[0053] In some embodiments, the phase and frequency offset
variations in one vector is compensated in the fine carrier
recovery component by processing data in smaller vectors. The fine
carrier recovery component 670 may serialize the operation of phase
and frequency offset correction on the adaptive equalizer output
614. To satisfy this, parallel carrier recovery 670 blocks may
operate on different equalizer outputs. For example a first fine
carrier recovery component may operate in a serial manner on the
equalizer output 614 provided at a first time, while a second fine
carrier recovery component may operate on the equalizer output 614
provided at a second, subsequent time, while the first carrier
recovery component is busy with the current equalizer output. The
fine carrier recovery component 670 may therefore include multiple
carrier recovery sub-components operating in parallel. A
demultiplexer may sequentially connect the equalizer output 614 to
each sub-component in turn, or a queue of equalizer output blocks
can be stored and operated on by carrier recovery sub-components as
capacity permits.
[0054] In the prior art of the decision directed equalizers, the
carrier recovery component (e.g. 420 in FIG. 4, comparable to
component 670 of FIG. 6) is placed in the adaptive equalizer loop.
In this configuration and to meet equalization performance
requirement, the delay in the loop has to be tight, which limits
the ability of perform more signal impairments correction, such as
DC nulling and skew correction, before providing the data to the
carrier recovery component. In the current embodiments, the carrier
recovery component 670 is disjoint from the adaptive equalizer
loop. Therefore, adding more signal impairments correction before
the component 670 is feasible without affecting the equalizer loop
delay. The signal impairment correction blocks, such as DC nulling
and skew correction, may improve the performance of the carrier
recovery component 670. In some embodiments, therefore, one or more
signal correction operations, such as DC nulling and skew
correction, are performed on the equalizer 614 prior to its
provision to the carrier recovery component 670, or these signal
correction operations may be performed by the carrier recovery
component 670, for example prior to other carrier recovery
operations.
[0055] Having reference now to FIG. 7, an embodiment of a feedback
component 700, usable as the feedback component 630 of the
apparatus 600, will now be described. The feedback component 700
includes an initial carrier recovery component 720 configured to
generate a carrier recovery output 722 based on the equalized
output 614 from the adaptive equalizer and an estimate 717 of
frequency components of the received signal. The carrier recovery
output is used at least to inform the phase rotation and phase
de-rotation operations. In some embodiments, the estimate 717 is
received from the carrier recovery component, and may be part of
the data 617 received therefrom. In other embodiments, the estimate
717 of frequency components may be generated internally by the
initial carrier recovery component 720. In some embodiments, the
initial carrier recovery component can also provide output data
such as estimates 716 of phase offset of the received signal. The
initial carrier recovery component 720 may contain an integrator
(also referred to as a ramp or ramp function, see e.g. ramp 830 in
FIG. 8) that integrates the frequency offset estimate 717. The
component 720 may also contain a numerical controlled oscillator
(NCO) (see e.g. NCO 834 in FIG. 8) that may generate a sinusoidal
signal with phase proportional to the sum of estimate of phase
offset 716 and the integral of the estimate of frequency offset. In
various embodiments, the carrier recovery output 722 is the NCO
output. The NCO outputs a sinusoidal signal with a changing phase.
The phase rotator (see e.g. phase rotator 842 in FIG. 8) within the
component 740 may be a multiplier that multiplies the adaptive
equalizer output 614 with the NCO output 416. Also a phase
de-rotator (see e.g. phase de-rotator 850 in FIG. 8) within the
component 740 may be implemented as a multiplier with the conjugate
of the NCO output to reverse the function of the rotator.
[0056] The feedback component 700 further includes a phase rotation
and detection component 740, which is configured to receive the
equalized output 614 from the adaptive equalizer. The phase
rotation and detection component 740 is further configured to
detect symbols carried in the received signal based on the
equalized output 614 following a phase rotation performed on the
equalized output. The phase rotation is performed (by the phase
rotation and detection component) based on the carrier recovery
output 722 generated by the initial carrier recovery component 720
and provided to the phase rotation and detection component 740. The
detected symbols are fed back 742 directly or indirectly to the
adaptive equalizer. For indirect feedback, the detected symbols are
subjected to a phase de-rotation (e.g. by a portion of the phase
rotation and detection component following a detector) prior to
feedback. The phase de-rotation may be equal in magnitude and
opposite in direction to the phase rotation.
[0057] Having reference now to FIG. 8, one particular embodiment of
the feedback component 700 of FIG. 7 will now be described. As with
FIG. 7, the feedback component 800 of FIG. 8 includes an initial
carrier recovery component 820 and a phase rotation and detection
component 840. In other embodiments, the illustrated internal
configuration of either or both of the initial carrier recovery
component 820 or the phase rotation and detection component 840 can
be varied.
[0058] The initial carrier recovery component 820 is configured to
receive the equalized output 614 from the adaptive equalizer and
generate an initial estimate 824 of phase offset in the received
signal based on the equalized output 614. The initial estimate 824
may be provided by an initial phase estimator 822. In some
embodiments, the initial phase estimate 824, or related output, may
also be provided to an external component, such as a fine carrier
recovery component. As such, the initial phase estimator uses part
or all of the adaptive filter output to determine (e.g. calculate)
an initial phase offset. The initial phase estimator 822 may
operate by comparing phase offset in received pilot sequence in 614
with original known pilot sequence to estimate phase offset 824 and
may also estimate the initial frequency offset.
[0059] In some embodiments, the initial phase estimator 822 may use
one or more training sequences in the equalized output 614 to
estimate the phase offset. The component 840 uses this estimated
phase offset with the frequency offset to rotate equalizer output
614.
[0060] The initial carrier recovery component 820 is further
configured to generate a further indication 832 of phase offset in
the received signal based on an estimate 817 of frequency of
components of the received signal. The estimate 817 may be received
from another component of the receiver, such an external
(fine/final) carrier recovery component operating partially
disjointly from the initial carrier recovery component 820. In
other embodiments, the estimate 817 may be internally generated by
the initial carrier recovery component 820. In some embodiments, as
illustrated, the frequency estimate 817 (or related data), referred
to herein as w, is either estimated using a training sequence in
the equalizer output 614 or is based on information provided (817a)
from the fine carrier recovery component (or a combination
thereof). The frequency offset is typically relatively slowly
changing with time, so the frequency estimate from the fine carrier
recovery component can be used in initial carrier recovery
component, even though the data processed in the fine carrier
recovery component is older than the data currently being processed
in the initial carrier recovery component. The frequency estimate w
may include one value for each equalizer output, and it is passed
to a ramp function 830 which essentially integrates the frequency
estimate 817 to generate the further indication 832 of phase offset
wt. In a Typical Implementation, the ramp component actually
creates a discrete time series of wt that is equal in length to the
width of the equalizer output bus 614. The output of the ramp
component is a discrete sequence wnT, where T is the sampling
period and n is the time order index of the samples in the
equalizer output 614. The initial phase estimator component may use
training sequences in the equalizer output data to estimate the
phase offset as one value. Referring to the initial phase offset
824 as {circumflex over (.theta.)}, the NCO component 834 adds the
initial phase estimate {circumflex over (.theta.)} to the ramp
output time sequence 832 and generates a sinusoidal signal 838 that
has constant phase {circumflex over (.theta.)} and constant
frequency w with width equal to the equalizer output bus width. The
generated sinusoidal signal can be denoted exp(j(wnT+{circumflex
over (.theta.)})), where j is {square root over (-1)}. The
sinusoidal signal 838 or the complex conjugate of it is used to
rotate or de-rotate equalizer output 614 or detector output 848.
The output of the NCO is a sinusoidal signal with sinusoidal phase
changing with the phase offset estimate and the integration of the
frequency offset estimate.
[0061] The initial carrier recovery component 820 is further
configured to generate a carrier recovery output 838 based on the
initial phase estimate 824 and the further indication 832 of phase
offset. The initial estimate 824 of phase offset may be an estimate
of phase offset at initial time, which used as an estimate for the
input vector 614. The exact phase offset per symbol correction is
determined during the fine carrier recovery operation. For example,
in some embodiments, as illustrated, a numerically controlled
oscillator (NCO) 834 receives the initial estimate 824 of phase
offset as well as the further indication 832 of phase offset. Based
on these, the NCO 834 generates the carrier recovery output 838.
The initial phase offset 824 and the integral of frequency offset
832, or other further indication of phase offset(also referred to
as the phase offset vector) are used to generate a carrier recovery
output 838 for phase-rotating the adaptive equalizer output.
Generating the carrier recovery output may include operating an NCO
based on the initial estimate of phase offset and the integral of
frequency offset.
[0062] In more detail, in various embodiments, the initial estimate
824 of phase offset is added to the phase offset vector (i.e.
integral of frequency offset 832) and the sum is used as the amount
of phase rotation, in the carrier recovery output 838, to be
applied to the adaptive equalizer output.
[0063] In some embodiments, the initial carrier recovery component
820 is further configured to provide an estimate of phase offsets
in the received signal to a fine carrier recovery component of the
receiver. As mentioned elsewhere herein, the fine carrier recovery
component is separate from the initial carrier recovery component
and the phase rotation and detection component.
[0064] The phase rotation and detection component 840 comprises a
phase rotator 842, a detector 846, and, in some but not necessarily
all embodiments, a phase de-rotator 850. The phase rotation and
detection component 840 receives the equalized output 614. The
phase rotator 842 is configured to perform a phase rotation on the
equalized output 614 based on the carrier recovery output 838
received from the initial carrier recovery component 820. For
example, the phase rotator may rotate the equalized output 614 with
the sum of initial phase offset 824 and the phase offset vector
(i.e. integral of frequency offset 832). The detector 846 receives
output 844 from the phase rotator 842 and is configured to detect
symbols carried in the received signal based on the equalized
output following the phase rotation. That is, the output 844 of the
phase rotator 842 is passed through the detector 846. The detector
is further configured to provide a detector output 848 indicative
of the detected symbols. The phase de-rotator 850 is configured to
perform a phase de-rotation on the detector output 848. The phase
de-rotation opposes the phase rotation performed by the phase
rotator 842, i.e. such that the phase de-rotation is equal in
magnitude and opposite and direction to the phase rotation. The
phase de-rotation is also performed based on the carrier recovery
output 838 of the initial carrier recovery component 820. The phase
de-rotated detector output 852 is fed back to the adaptive
equalizer, where it is used in a process for updating the
coefficients of the adaptive filter thereof. The output of the
detector is therefore used by the adaptive equalizer, for example
by an error signal calculator thereof, to calculate an error signal
for use in updating the adaptive filter coefficients. For example,
the error signal can be passed to a coefficient update calculator
and used thereby to update the adaptive filter coefficients.
[0065] In some embodiments, the phase rotation and detection
component 840 is configured to provide the equalized output 844
following the phase rotation as symbol input to a separate fine
carrier recovery component. This alternative arrangement allows the
fine carrier recovery component to benefit from the
already-performed initial phase rotation by the phase rotator
842.
[0066] Operation of another embodiment of initial carrier recovery
block 820 is illustrated in FIG. 13. In this embodiment, the ramp
component 830 integrates the estimate of frequency components 817
that may be calculated in and provided by the fine carrier recovery
component, or in and provided by the initial carrier recovery
component using a training sequence in the equalizer output 614.
The estimate of the frequency component 817, referred to herein as
0, may be integrated by the ramp component 830. The output 832 of
the ramp component is the phase sequence wnT that may be used by
the NCO component 834 to generate a sinusoidal signal output 835
that can be denoted by exp(j(wnT)). The output 835 of the NCO may
be provided to and used by the frequency rotator component 1238 to
rotate the output of the equalizer 614. The frequency rotator
component 1238 may comprise a multiplier that multiplies the
equalizer output with the NCO output sinusoidal sequence 832. The
output 1239 of the frequency rotator component 1238 is fed to both
the initial carrier recovery component 820 and the phase rotation
and detection component 840. In this embodiment, the frequency
offset is corrected first using the frequency rotator component
1238 and the phase offset is estimated afterward in Initial Phase
Estimation component 822. By correcting the frequency offset first,
the initial phase estimation component 822 may or may not use
training to estimate the phase offset. The initial phase estimation
component 822 may test pre-selected phase offset values and choose
the phase offset that minimizes mean square error in the frequency
rotator component output 1239. The output of the initial phase
estimation component 822 (i.e. the initial phase estimate 1224) is
a single phase estimate used (by the phase rotation and detection
component 840) to rotate all the output 1239 of the frequency
rotator 1238. There is no need to have an NCO in the initial
carrier recovery component 820 in this embodiment because the
initial phase estimate is one value, referred to herein as
{circumflex over (.theta.)}, and the phase rotator component 842
may multiply all the outputs 1239 of the frequency rotator
component by exp(j{circumflex over (.theta.)}) which is one complex
phase. The phase de-rotator component 850 reverses the operation of
both the phase rotator component 842 and the frequency rotator
component 1238. The phase de-rotator component 850 adds the NCO
output sequence 832 to the phase offset exponent from the initial
phase estimation output 1224 to form the total phase offset
sequence denoted by exp(j(wnT+{circumflex over (.theta.)})) and
multiplies this sequence with the output 848 of the detector
component 846.
[0067] FIG. 9 illustrates the apparatus 800 of FIG. 8, integrated
with an adaptive equalizer 910 and a fine carrier recovery
component 930, along with a decoder 940 which receives data from
the fine carrier recovery component 930, in accordance with an
embodiment of the present invention. The apparatus 800 operates as
described above with respect to FIG. 8 to provide feedback to the
adaptive equalizer 910, and also to exchange data with the fine
carrier recovery component 930. As such, the apparatus 800 and the
fine carrier recovery component 930 operate in a partially disjoint
manner.
[0068] FIG. 10 illustrates the apparatus 800 of FIG. 8, integrated
with an adaptive equalizer 910 and a fine carrier recovery
component 930, along with a decoder 940 which receives data from
the fine carrier recovery component 930, in accordance with another
embodiment of the present invention. The apparatus 800 again
operates as described above with respect to FIG. 8 to provide
feedback to the adaptive equalizer 910. Rather than, or optionally
in addition to, exchanging phase and frequency estimates between
the apparatus 800 and the fine carrier recovery component 930, the
output 844 of the phase rotator 842 is provided to the input of the
fine carrier recovery component 930.
[0069] As illustrated in FIGS. 9 and 10, both the adaptive
equalizer 910 and the apparatus 800 may operate on blocks of N
symbols in parallel. The fine carrier recovery component 930 may
operate on M<N symbols in parallel. The decoder 940 may operate
on N symbols in parallel.
[0070] Further details regarding the adaptive equalizer 910 of
FIGS. 9 and 10 will now be provided. A worker skilled in the art
will readily understand general operation of the adaptive equalizer
itself. The adaptive equalizer includes an error signal calculator
912, a coefficient update calculator 914, and an adaptive filter
916. The error signal calculator is configured to calculate the
error between the output of the adaptive filter 916 and the output
of the detector 846, for example following phase de-rotation (i.e.
output 852). The phase de-rotation phase-aligns the signals
received by the error signal calculator so that a proper comparison
can be performed. The coefficient update calculator 914 updates the
coefficients for use by the adaptive filter based on the error
signal provided by the error signal calculator 914.
[0071] In various embodiments, adaptive filter coefficient updating
can be performed in accordance with the following equation:
W(n+1)=W(n)+J(e(n),X(n)), where W(n) is the adaptive filter
coefficient vector,e (n) is the error vector, X(n) is input of the
adaptive filter and J(.) is a function of the error vector and/or
filter input.
[0072] The fine carrier recovery component 930 operates similarly
to the carrier recovery component 420 of FIG. 4. However, the fine
carrier recovery component 930 may omit a phase de-rotator.
Further, the fine carrier recovery component 930 may transmit data
to the initial carrier recovery component 800, receive data from
the initial carrier recovery component 800, or both. With the Fine
carrier recovery component 930 being separate from the equalizer
loop, there is freedom in complexity level of the design of this
component. Also more impairment correction operations (or
corresponding components) can be implemented before operation of
the fine carrier recovery component 930 to improve the phase
estimate provided thereby. These impairment correction operations
may be integrated in the carrier recovery component 930 or
performed on the filter output 614 prior to the input to the fine
carrier recovery component 930, either before or after
serialization.
[0073] FIG. 11 illustrates a method 1100 for operating an adaptive
equalizer, in accordance with an embodiment of the present
invention. This method corresponds to operation of the adaptive
equalizer and embodiments of the feedback component as described
above. The method includes filtering 1102 a block of input data
using the adaptive filter, where the adaptive filter is implied
using a current set of filter coefficients. The method further
includes determining 1104 if the data block output by the adaptive
filter includes training data. If so, the method further includes
estimating 1106 phase and frequency offsets in the received signal
based on the training data. In this case, the method also further
includes integrating the frequency estimate and adding the integral
output to the phase offset estimate.
[0074] The method further includes rotating 1108 the output of the
adaptive filter to correct for phase and frequency offsets. If the
data block included training data, the rotation is based on the
current results obtained from operation 1110. Otherwise, the
rotation may be based on other data, such as the results obtained
from the most recent previous iteration of operation 1110. The
method includes subsequently detecting 1112 originally transmitted
symbols in the data block based on the phase-rotated output of the
adaptive filter. The method includes subsequently de-rotating 1114
the detected symbols, that is, performing a phase rotation on the
detected symbols that is equal in magnitude and opposite in sign to
the phase rotation applied to the adaptive filter output in
operation 1108.
[0075] The method further includes, for example by suitable
components of the adaptive equalizer, calculating 1116 an error
between the adaptive filter output and the de-rotated detected
symbols provided by operation 1114. The method further includes,
for example by the components of the adaptive equalizer, updating
1118 the adaptive filter coefficients based on the calculated
error.
[0076] FIG. 12A illustrates a method 1200 for operating a signal
receiver, according to an embodiment of the present invention. It
is noted that, in some embodiments, only parts of the illustrated
method are provided. The method includes generating (or otherwise
providing) 1205, by an adaptive equalizer, an equalized output
indicative of a corrected version of a received signal. The
equalized output is generated using currently available filter
coefficients. The method further includes receiving 1210, by a
feedback component, the equalized output from the adaptive
equalizer. The method further includes providing 1215 feedback from
the feedback component to the adaptive equalizer, based on the
equalized output. This feedback is usable for updating the filter
coefficients. The method may further include directly receiving
1220 the equalized output from the adaptive equalizer by a fine
carrier recovery component separate from the feedback component.
The method alternatively includes indirectly receiving 1222 the
equalized output from the adaptive equalizer by the fine carrier
recovery component. That is, the feedback component can send the
equalized output to the fine carrier recovery component, possibly
after some modification such as initial phase rotation. The method
further includes providing 1225, by the fine carrier recovery
component, estimates of symbols carried in the received signal
based on the equalized output.
[0077] In some embodiments the method 1200 further includes one or
both of: providing at least a portion of data 1230 generated by the
feedback component to the fine carrier recovery component; and
providing at least a portion of data 1232 generated by the fine
carrier recovery component to the feedback component. For example,
in some embodiments, the method further includes, by the feedback
component, performing an initial phase rotation 1228 on the
equalized output and providing the equalized output following the
initial phase rotation as the data 1230 to the fine carrier
recovery component.
[0078] Having reference now to FIG. 12B, in some embodiments, the
method 1200 further includes, by an initial carrier recovery
sub-component (ICRC) of the feedback component, generating 1240 a
carrier recovery output based on the equalized output received
1210a from the adaptive equalizer and an estimate of frequency
components of the received signal. The carrier recovery output
informs the phase rotation operation. For example, the carrier
recovery output may be a sinusoidal signal with a particular phase,
generated by an NCO. The estimate is received 1238 from the fine
carrier recovery component. The method further includes, by a phase
rotation and detection (PRDC) sub-component of the feedback
component, receiving 1210b the equalized output from the adaptive
equalizer. The PRDC also receives 1241 the carrier recovery output
from the ICRC. The method further includes, by the PRDC, performing
1244 a phase rotation on the equalized output. In some embodiments,
the phase-rotated output can be provided 1245 to the fine carrier
recovery component. The method further includes, by the PRDC,
detecting 1246 symbols carried in the received signal based on the
equalized output following the phase rotation. The phase rotation
is performed based on the carrier recovery output generated 1240 by
the initial carrier recovery component. The detected symbols may be
fed back 1215a directly to the adaptive equalizer. Alternatively,
the method may further include performing a phase de-rotation 1248
on the detected symbols, and feeding back 1215b the detected
symbols indirectly to the adaptive equalizer, that is, following
the phase de-rotation 1248.
[0079] Having reference now to FIG. 12C, in some embodiments, the
method 1200 further includes the following operations, performed by
the initial carrier recovery sub-component (ICRC) of the feedback
component. The method again includes receiving 1210a the equalized
output from the adaptive equalizer and receiving 1238 an estimate
of frequency of components of the received signal. The method
includes generating 1252 an initial estimate of phase offset in the
received signal based on the equalized output. The method includes
generating 1254 an integral of frequency offset in the received
signal based on the received estimate of frequency of components.
The generation 1254 may be performed by an integration operation,
for example using a ramp function. The method includes generating
1240 the carrier recovery output based on the initial estimate of
phase offset and the integral of frequency offset.
[0080] As will be readily understood by a person skilled in the
art, embodiments of the present invention can be implemented using
a variety of electronic devices. For example, various components
can be implemented using a processor such as a digital signal
processor, an application specific integrated circuit (ASIC), or a
field programmable gate array (FPGA), or a combination thereof. A
processor may execute program instructions stored in memory in
order to carry out operations as described herein. However, it is
considered that the use of dedicated electronic devices such as
integrated circuits may be more suitable for at least some
practical embodiments. One or a combination of integrated circuits,
for example driven by a clock, can be configured during or
following manufacture to implement the various operations and
components described herein. In some embodiments, optical signals
received via an optical transmission line can be converted into
electronic signals via suitable detectors, and converted into
digital form using analog-to-digital converters. The digitized
signals are then received and processed by other components as
described above. Each of the various components described herein,
such as the adaptive equalizer, feedback component, fine carrier
recovery component, and sub-components thereof, such as the initial
carrier recovery component, phase rotation and detection component,
phase rotator, phase de-rotator, detector, initial phase estimator,
ramp, NCO, decision component, phase and frequency offset detector,
etc. can be implemented using such electronic devices. Multiple
components can be partly or fully provided using the same
electronic device, different components can be provided using
different operatively coupled electronic devices, or a combination
thereof. Methods as described herein can similarly be implemented
using such electronic devices.
[0081] In some embodiments, for example for prototyping purposes
and provided that sufficient processing capability is available,
the present invention can be implemented partially or fully using a
computer processor executing instructions provided in software.
Similarly, in some embodiments, the present invention can be
implemented in firmware, using an electronic device executing
program instructions provided as part of the firmware. However, it
is noted that software and firmware implementations may not have
sufficient speed at this time for at least some practical
applications.
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